Interstate Traveling Bus Lines from IL to SD help!?

My niece is going to visit family this summer in South Dakota and I need to find a bus line that will take her there, but Jefferson does not sell tickets for unaccompanied minors and I don't want to put her on Greyhound. I need a list of bus lines that might take her to Aberdeen or any area around Mobridge, McLaughlin, or Pierre (not including Greyhound or Jefferson).